BUSTED: Former cop Drew Peterson has been charged in the death of his third wife, Kathy. (AP)

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When she went missing, her boyfriend told the cops she just walked off on her own, left him for her own reasons, even though she was a mother and had left all of her personal effects behind. Her family told the cops: No, she was murdered and he did it. Years later, everyone who loved her is convinced the cops will never charge him.

That woman, Christie Cales, disappeared on March 11, 1998, last seen in Blue Island, Ill.

Nine years later, on Oct. 28, 2007, her daughter, Stacy Cales, also went missing. Stacy’s husband told the cops she just walked off on her own, even though she was a mother and had left all of her personal effects behind. Her family told the cops: No, she was murdered and he did it. Years later, everyone who loved her is convinced the cops will never charge him.

In the latter case, the “him” is Drew Peterson, the former police sergeant currently on trial for the murder of his third wife, Kathleen Savio, in March 2004. But his trial is equally about his fourth wife, Stacy, who has never been found.

“This is still an open wound,” says Norma Peterson, sister-in-law of Drew and a close friend of Stacy. “It’s been a long time. We’ve been caught up with Kathy, and so you put those feelings [about Stacy] on hold. We have not dealt with it at all.”

With a case like this — or any number of tabloid-ready homicides in which young, beautiful women are murdered by charismatic, attractive husbands — the Geraldos and the Nancy Graces and the true-crime shows all fall back on one familiar trope: How could this ever have happened? How could a girl so pretty and smart, with so many options, fall for a guy like that?

The uncomfortable truth is for every girl who marries a truly dangerous man, there are dozens more who came before and stayed away, who could sense something wasn’t right. And when you begin to ask questions about Stacy Peterson’s life before Drew, you can very quickly understand why she wound up married to someone like him.

On Jan. 20, 1984, Stacy Ann Cales has the great misfortune to be born to Anthony and Christie Cales, two people with more interest in partying than parenting — even though they would have five children: daughters Jessica, Stacy, Cassandra and Lacy and son Yelton. Together, they were also raising Tina, Christie’s daughter from a previous relationship.

Six weeks before Stacy was born, the Cales’ ranch house in Downers Grove, Ill., caught fire. Christie, then about 8 months pregnant, barely escaped. Her 2-year-old daughter, Jessica, did not; her body was found behind the living-room couch, as if she had been hiding.

Christie and Anthony began fighting and drinking harder than ever. Christie said Anthony, who was not in the house the night of the fire, yelled, “You bitch, you burned my baby!” and later waved a .357 Magnum in her face.

Christie’s brother, Kyle Toutges, told The Post the blaze had been started by children playing with matches in the Cales home.

The kids “had a lot of trauma,” says Cheryl Weiser, who had a son, now 22, with Christie’s brother Kyle. “There were drugs, alcohol and physical abuse in that family. They were good kids without direction or security.”

Despite their violent marriage, issues with drugs and alcohol, and neglect and abuse of their children, Anthony and Christie decided to have more of them, and on June 10, 1985, Christie gave birth to another girl, Cassandra. Two years later, another girl, Lacy, followed. Lacy died of SIDS in October 1987.

Friends and family members who spoke with The Post say Anthony and Christie never recovered from Lacy’s death, and the surviving kids were on their own.

“We used to see Stacy running around here without any clothes except her diaper in the middle of winter,” says Rowland Henderson, who has lived around the corner from the Cales’ then-home for 50 years. “Her mother would be sleeping on the couch, and Stacy would run out the front door.”

By now, Christie would disappear for weeks at a time. “My dad would just say, ‘She ran off,’ ” says Stacy’s sister, Cassandra. She doesn’t recall her father ever calling the cops and to this day has no idea where her mother was or what she was doing.

Records show Christie was in and out of psych wards, and she copped to drinking a case of beer a day. She was busted for shoplifting booze and cigarettes and in 1990 arrested for drunken driving.

Anthony was not much better. “The father medicated his pain with liquid spirits,” says Christie’s brother Kyle. “That’s all he did. He had a $30,000 car, and the kids were eating mac and cheese. He had them on a budget.”

By 1990, the couple had two foreclosure suits filed against them, and Anthony filed for divorce. Records show Christie initially contested the filing, but after repeatedly missing court dates, Anthony was awarded sole custody of Yelton, Stacy, Cassandra and Tina, by now 14.

The neighbors watched in horror as Anthony, who alternately worked in construction and animal control, brought home raccoons or stray animals that he would neglect to feed or shelter.

“They had dogs that they would leave out all winter until they died,” the longtime neighbor says. “One day they were barking, the next they were not.”

Somehow Stacy emerged from the crucible of her childhood a kind and loving person, determined to forge a better life for herself. She made friends easily and graduated high school early, at 16.

That was 1998, the same year Stacy’s mother, Christie, went missing. She was last seen walking along a street at 4:30 in the afternoon in the Chicago suburb of Blue Island. Cassandra says she, Stacy and Tina knew their mother had been murdered.

“We knew the guy she was living with did it,” a man her children knew little about, Cassandra says. How? “Because he said she left for church and never came back, but she left her purse.”

Cassandra went to the cops. “They wouldn’t listen to us,” she says. “They brushed it off, never looked into it. That’s when things got really hard.”

Stacy was devastated. Friends and neighbors say she never talked about her mom after that.

“It just never came up,” says neighbor Mary Beth Cesare, whose then-teen son was friends with Stacy and Tina. “They never let on what their life was like.”

Stacy dreamed of becoming a nurse but, without the money for college, took a series of odd jobs.

In 2001, she was 17, working as a desk clerk at a hotel in Bolingbrook, Ill.

That’s where she met a local cop on patrol named Drew Peterson.

At the time, Peterson was 47 years old and married to his third wife, Kathleen Savio. He was, according to his first two wives, an abusive adulterer. But Stacy saw an older man who would protect and take care of her.

“She was infatuated with him because he was a father figure,” says Stacy’s aunt Cheryl. “He was secure, and he had charisma. Stacy wanted a better life.”

Peterson divorced Savio in 2001; he and Stacy were married in October 2003, three months after the birth of their first son. Stacy named him Anthony, after her father.

Stacy was overjoyed. She was a wife, a mother, and she reveled in domesticity: cooking, cleaning, decorating Anthony’s room. Her marriage, she believed, was good, even if her husband could be a little controlling, or refuse to watch the baby so she could take her exam to get into nursing school, or let her get a job, or call relentlessly whenever she was out of the house.

“We would go to Walmart, and he would call her,” says Sharon Bychowski, a close pal. “We went to Kohl’s, and his squad car was out front. He would go to roll call and leave to find her whereabouts.”

When Drew’s ex-wife Kathy was found dead in her tub in 2004, it was Stacy who tried to convince Drew’s brother Paul and Paul’s wife, Norma, that Drew had nothing to do with it. “She just wanted to keep that family together,” Bychowski says.

By now, Stacy had another child, a 1-year-old named Lacy, like her tragic sister.

But Stacy’s life was, and would be, cratered with loss. Her mother was missing. Her brother, Yelton, had been in and out of jail since 2002. Her beloved sister, Tina, died of cancer in 2006.

It was around then, say family and friends, that Stacy began to re-evaluate her marriage.

“Mentally and physically, her life was changing,” says Bychowski. “She was only 23, but she lived like she was on fire to become a better person.”

Stacy started paying more attention to her appearance. She took her kids to Bible study. She began working as a sales rep for Avon.

None of these things sat well with her husband. Of course, there were many things she kept from him, including her increasing suspicions about what really happened to Kathy Savio. In October 2007, she found the courage to tell her husband she wanted a divorce.

Bychowski came by to check in on Stacy. “She was calm and cool, and she said, ‘I am already dead. He is going to kill me.’ ”

A week later, on Oct. 28, 2007, Stacy Ann Peterson vanished. Drew told authorities his wife had called him from the airport to tell him she was leaving him for another man.

Stacy’s family and friends pushed the cops, and so did Kathy Savio’s people. Savio’s body had been found dry and facedown in her empty bathtub, yet her death was ruled an accidental drowning. Finally, in 2008, Savio’s body was exhumed. The medical examiner ruled her death a homicide, though the state’s attorney’s office would not disclose evidence that led to the finding.

Peterson was arrested in 2009 and has been jailed on a $20 million bond ever since. Rob Lowe played him in the Lifetime movie.

Ironically, the Peterson case has prompted Blue Island authorities to pursue the disappearance of Stacy’s mom with renewed vigor. “The cops swabbed me for DNA last week,” Cassandra Cales says. She can barely hide her disdain.

Stacy’s case remains open, with cops still searching for her remains.

Stacy herself rarely talked about her mother’s vanishing, but her sister-in-law Norma says Stacy did bring it up once.